A hot strip mill for rolling a hot metal workpiece generally includes a roughing mill and a finishing mill. In such a case, a roll gap of the finishing mill and a rolling speed thereof have been set by calculations such that the size and temperature of rolled metal at a delivery side of the finishing mill become as required.
Such conventional setup calculations will be described in brief with reference to FIG. 5.
A roughing mill RM in FIG. 5 comprises a single mill stand having reversible rolls and a hot metal or a hot bar, which after having been rolled an odd number of times by the roughing mill, is supplied to a finishing mill FM composed of a plurality of mill stands F.sub.l. . . , F.sub.n. In the finishing mill, the metal is rolled sequentially by the respective stands to obtain a rolled metal having predetermined size.
In general, the roll gaps and rolling speeds of the respective stands of the finishing mill are determined preliminarily before the hot metal from the roughing mill is supplied to the respective stands F.sub.i (i=1, 2, . . . , n). After the hot metal enters into the respective stands, the thickness of the hot metal from the respective stands F.sub.i are maintained at the predetermined values under the control of an automatic gauge control device. Therefore, the setup calculations for the finishing mill are performed for a top end portion of the metal to minimize off-gauge portions. The term "top end portion" means a portion of the metal which is inside an actual top end of the plate by a distance of several meters.
In the setup calculations for conventional finishing mill, the temperature T.sub.RD of the top end portion of the metal in the final pass by which the metal leaves the roughing mill RM is detected by a pyrometer RDT provided on a delivery side of the roughing mill at a time 1 shown in FIG. 5. Then, the metal is transported on a delay table arranged between the roughing mill RM and the finishing mill FM to a position on the entry side of the finishing mill FM in which a pyrometer FET is provided. The temperature T.sub.FE of the top end portion of the metal is measured by the pyrometer FET at a time 2. The temperature of the metal of which the top end portion passes through the respective stands F.sub.i of the finishing mill FM are estimated preliminarily at a time 1 when the temperature T.sub.RD of the top end portion of the metal is Obtained. In FIG. 5, the metal temperature T.sub.1 is estimated at a time 3 when its top end portion enters into the first stand F.sub.1. The conditions of a scale breaker FSB are then considered. In this manner, the roll gaps of the respective stands F.sub.i are set on the basis of the metal temperature at the respective stands F.sub.i in such a way that the thickness of the metal at the delivery side of the respective stands becomes the predetermined values. Furthermore, the roll speeds at the respective stands F.sub.i are set at a time when the temperature T.sub.FE of the top end portion of the metal is obtained by taking the temperature drop in the finishing mill FM into consideration so that the temperature T.sub.FD of the top end portion of the metal passing through the pyrometer FDT provided in the delivery side of the final stand F.sub.n at a time 4 becomes the objective temperature.
It is a recent tendency that a coiler having no mandrell, referred to as a coil box, is arranged on a delay table between the roughing mill and the finishing mill in order to improve the space economy, to reduce the amount of skid marks formed in a heating furnace provided upstream thereof and to minimize the energy consumption, etc.
In such a coil box, the top end and the tail end of the bar are reversed by winding and rewinding, and the temperature variation of the wound bar is substantially different from that on the delay table.
It is impossible to directly apply the conventional setup of the finishing mill to such a construction of the hot strip mill. A typical example of a temperature estimation of metal in a finishing mill having a coil box is disclosed in U.S. Pat. No. 4,068,511. According to the temperature estimation method disclosed in this patent, the delay table is divided into a coil box portion and the remaining portion and the bar temperature at an entry side of the finishing mill is obtained by sequential calculation of the temperatures of respective portions using the bar temperature at a delivery side of a roughing mill as an initial value. Defects of this method are that, due to the fact that the initial temperature is a detection value from the pyrometer RDT disposed on the delivery side of the roughing mill, i.e., a surface temperature of the bar, it is necessary, in order to minimize the estimation error in the calculation, to repeat the calculation at constant intervals, and it is also necessary to prepare a heat loss compensation table to estimate the temperature drop in the coil box and to repeatedly refer to the table for every calculation.